With the advent of high temperature excitation sources for analysis by optical atomic emission spectrometric methods, for example, Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES), there has been a growing interest in the direct analysis of solids. This approach would bypass the tedious, time consuming and error-prone sample dissolution step involved in conventional analytical systems. However, the development of a practical method to introduce solids in a manner suitable for analytical purposes has been impeded by several major technological problems. The sample must be introduced uniformly so that the variations of the plasma characteristics are minimal. It is desirable to have a uniform sample delivery to satisfy the requirements of data acquisition techniques incorporated with most of the commercially available spectrometers. A controlled delivery of the sample is important to avoid overloading of the atomization source. It would be desirable to introduce a known amount of material within a desired time period. For a representative analysis the system should be able to introduce particles of a wide range of sizes and matrices without segregation according to size or density. Also, most systems require that the sample be transported in an inert gas.
There are several possible approaches to analyze solid materials without acid digestion, dissolution or other chemical processing to yield a liquid sample. Methods based on solid/liquid slurry nebulization, electrothermal vaporization, laser ablation, direct sample insertion have been proposed for analysis of solids. Methods based on the introduction of powders by the formation of a fluidized beds and aerosols has also been proposed to meet some of the analytical requirements.
A limitation of most of the systems previously described is that the analytical signals resulting from the introduction of the solid sample is transient; the mass of solid and the time duration of sample delivery is poorly controlled, or not controlled at all (e.g. laser ablation).
In U.S. Pat. No. 4,836,039 the present inventors have disclosed a method to generate a substantially uniform flow of particles by making use of a combination of mechanical agitation of the container and a flow of a gas for the generation of a fluidized bed.
However, none of the present systems provide all the features desired for solid sample delivery. Specifically, it would be desirable to be able to deliver a known mass of material within a known period of time, and to provide a constant feeding rate from the beginning to end of the sampling period. Furthermore, it would be desirable to be able to provide controlled delivery by electronic means under computer control to obtain a predetermined particulate material delivery rate, predetermined sampling intervals, etc.